Proton exchange membrane fuel cells (PEMFCs) convert reactants, namely fuel (such as H2) and oxidant (such as O2 or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). MEA durability is one of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For automotive application, an MEA is required to demonstrate durability of about 6,000 hours.
The membrane serves as a separator to prevent mixing of reactant gases and as an electrolyte for transporting protons from anode to cathode. Perfluorosulfonic acid (PFSA) ionomer, e.g., Nafion®, has been the material of choice and the technology standard for membranes. Nafion® consists of a perfluorinated backbone that bears pendent vinyl ether side chains, terminating with SO3H. The chemical structure of Nafion® is as follows:

Failure of the membrane as an electrolyte will result in decreased performance due to increased ionic resistance, and failure of the membrane as a separator will result in fuel cell failure due to mixing of anode and cathode reactant gases. The chemical degradation of PFSA membrane during fuel cell operation is proposed to proceed via the attack of hydroxyl (.OH) or peroxyl (.OOH) radical species on weak groups (such as a carboxylic acid group) on the ionomer molecular chain. The free radicals may be generated by the decomposition of hydrogen peroxide with impurities (such as Fe2+) in a Fenton type reaction. In fuel cells, hydrogen peroxide can be formed either at Pt supported on carbon black in the catalyst layers or during the oxygen reduction reaction. The formation of hydrogen peroxide, generation of free radical, and degradation of the membrane are depicted in the scheme of FIG. 1.
The hydroxyl radical attacks the polymer at unstable end groups to cause chain zipping and/or could also attack an SO3− group under dry condition to cause polymer chain scission. Both attacks degrade the membrane and eventually lead to membrane cracking, thinning or forming of pinholes. The membrane degradation rate is accelerated significantly with increasing of the operation temperature and decreasing inlet gas relative humidity (RH).
Additive technologies have been applied to reduce membrane degradation in fuel cells. Additives studied included metal chelating agents, antioxidants, free radical scavengers, catalysts for decomposition of hydrogen peroxide, and combinations thereof.
The Japanese patent application JP 2008077974 A discloses electrodes for fuel cells in which the catalyst particles and the conductive carrier of the electrode catalyst layer are covered with a chemical compound consisting of nitrogen-containing 6 membered rings.
What is needed is an improved additive technology that provides additional resistance of MEAs, and specifically PFSA membranes of the MEAs, to degradation, resulting in improved MEA durability and performance under low RH in a fuel cell.